Carbon Credit

The word “carbon” is often used as a shorthand for carbon dioxide. Environmentalists talk of “carbon footprints” and “carbon pollution” and have injected such terms into the language. But not too long ago, we all recognized that Earth’s biosphere was one of “carbon-based life.” That is, of course, still true. But humans are changing the picture a little bit:

The Carbon Biosphere

How do trees get so big? A 75-foot oak tree might weigh fifteen tons. Where does the mass come from? Almost half is water; we subtract that and get about eight tons of “dry mass” for this oak tree. Some is nutrients pulled from the soil and other elements such as oxygen and hydrogen incorporated into the tree’s molecules, but these account for only about half of the tree’s weight. The rest is from the tree building its structure using carbon (C) from CO2, releasing the O2 back into the atmosphere.

It varies somewhat by species, but in general plants (large or small) are 45% to 50% carbon by dry weight, with this carbon almost entirely pulled from the air. There are various other processes involved, from the inclusion of animal life, decomposition after death (which releases some carbon), and incorporation of carbon into soils. But “just under half of the dry weight of the biosphere is carbon” is a pretty good rule of thumb.

Small Slavery

Many don’t appreciate that the normal metabolism of plant cells is like that of animal cells: They need to take in oxygen, and they produce CO2, just like animal cells. This is common to almost all life, with exceptions such as anaerobic bacteria. But plants also have photosynthesis going on, which is slave labor performed by bacteria the plant cells captured and domesticated a billion-plus years ago. This probably got started by a cell ingesting a photosynthetic bacterium it could not digest nor kill, and wound up benefiting from the bacterium’s byproduct. The master and the slave have both evolved since then.

We now call those slave cells chloroplasts, but they still have much of their original DNA from the time they were free-living bacteria. The parent plant cell cannot “make” chloroplasts; they still reproduce like bacteria (and are “raised” by the parent plant cell sort of like tiny farm animals) and then inherited by the daughter cells when the parent plant cell divides. Plant cells often have dozens of these slaves each; a wheat leaf cell might have 100 or so.

Oceanic algae, the cells that are the ancestors of plants and which first captured cyanobacteria and forced them into slave labor extracting carbon from the air, usually have only a single slave chloroplast each. Primitive early farmers, so to speak, operating on an even smaller economy of scale. There are some sea creatures that have incorporated algae into their own bodies, and thus can benefit from the photosynthesis — kind of a three-layered slavery effect. Some corals, clams, and others have done this.

That photosynthesis process that slave chloroplasts perform produces much more oxygen than the plant or algae cell needs, and thus leaves an excess. But the carbon is put to use, and the remaining oxygen is pushed back into the environment. In plants, access to the atmosphere is through the plant’s leaf openings called “stomata.” The stomata (singular “stoma”) consist of pairs of cells around a central port into the leaf’s interior. They open during the day to facilitate atmospheric gas exchange. Here are examples:

Aspects of the stomata change based upon the nature of the atmosphere, and they can fossilize, telling us something about the environment during the plant’s life.

Our current 21% or so of oxygen is essentially a billion-plus years of photosynthesis waste gas, discarded after the carbon is extracted from CO2. Life makes use of the oxygen, as it is highly reactive and thus valuable for chemical processes, but that also makes it somewhat toxic; it’s a bit like society’s relationship with nuclear fuel. And increasing the oxygen percentage brings on more damage more quickly. Note that a whole group of protective substances are called “antioxidants.”

Dying from Lack of Pollution

As algae expanded across the oceans, and ultimately colonized the land as plants over those billion-plus years, they have have been very gradually starving themselves of CO2. The levels have declined as they took it out of the atmosphere and trapped it in their body mass, becoming part of the seabed and the rock strata.

This process gets modified from time to time as the cold ocean absorbs more, or a warm ocean outgasses some CO2 (a bit like a warm carbonated beverage), or volcanic or asteroid events make their own contributions. The long-term trend is downward, but there is not much left:

The last half-billion years, as land plants developed and spread, have been particularly bad. Plants were dooming themselves to slow suffocation, until humans came along and released some of the CO2 the plants had earlier sequestered. This is, on balance, a good thing! (Note that on the graph above, the entire rise of CO2 associated with man’s activities is not shown, but the increase would only be about the thickness of the line used to draw the CO2 level.)

Plants evolved in an atmosphere with 10 to 20 times more CO2 than we currently have, and it was even more than that for algae previously. It is amazing that they can still grow at all! But somewhere in the 150ppm range, growth stops and plants die. During glaciations, such as the tens of thousands of years ending 20,000 years ago, CO2 drops to around 200pp or a little less, and plant life suffers badly.

Isn’t it odd to think of a substance as a “pollutant” where, if two-thirds of it were removed, it would effectively end most life on the planet? This would be the case if our current 400 parts per million of CO2 were cut to 133ppm.

Practical Effects

In a recent post at WattsUpWithThat, a team at University of Madison states that soybean yields have been negatively affected by weather events due to climate change. They suggest that this damage to US soybean crops has been $11 billion over the past 20 years (1994-2013). While they recognize that crop yields are rising rapidly and setting new records, they ascribe this to genetic changes and advanced agricultural management, and indicate that temperature increases have caused rises in some cool areas but declines in warm areas. They specifically mention three states as being on a list of heat-depressed declining soybean yields: Ohio, Arkansas and Kentucky. But all three states posted record yields, and temperature records don’t support their narrative at all.

The authors seem reluctant to give any credit to increased CO2, but countless experiments have demonstrated strongly increased yields from CO2 increases in the 20th century. One nice aspect to the CO2-induced crop improvements: They are still true even in areas such as Africa where advanced agricultural techniques are often unavailable. And enhanced CO2 also improves crops’ ability to deal with water and heat stresses.